The invention relates to an apparatus for the initial and improved machining of steer cams, brake spanners of automobiles, carried out by a face milling cutter or by the surface of a grinding wheel, the surface being perpendicular to its own axis of rotation, e.g. by a pot-shaped grinding wheel, where the profile of the brake spanner is mainly involute or the spiral of Archimedes.
An apparatus for the machining of the active surface of steer cams and brake spanners is known, in which a face milling cutter is employed as a tool. Such an apparatus is described on pages 36 and 37 of number 1978/5 of the Soviet monthly "Vestnik masinostroenija".
According to this known method of machining, the axis of rotation of the face milling cutter machining the active surface of the work piece is perpendicular to the axis of rotation of the work piece. The axis of rotation of the face milling cutter is parallel with the plane of the guide of the main headstock and is generally cross relative to the axis of rotation of the work piece.
The spindle driving the work piece is connected to a cam shaft through a gear drive of a one-to-one gear ratio. A master cam is located on the cam shaft on which the main headstock displaceable along a guide is pressed by spring. The shaft of the main spindle and the shaft of the face milling cutter secured to the main spindle, respectively, are parallel with the direction of movement of the main headstock. During one rotation of the cam shaft the face milling cutter machines a profile (active surface) of the work piece being identical with that of the master cam, according to the principle of mechanical copying.
A serious disadvantage of the known method lies in that the level of the technical implementation of the apparatus extremely falls behind the technical level of the method the apparatus is designed for. It is also noted that the surface machining principle of the method that can be carried out on the known apparatus by a face milling cutter is among one of the most advanced methods throughout the world, due to its low first-cost price.
Examining the drawbacks of the known apparatus individually, they are as follows:
the load carrying capacity of the master cam is limited, because it is in contact with the driven mechanism along a line, PA1 the profile of the master cam can only be produced expensively and with a limited accuracy, PA1 different master cams are required to machine each separate steering cam, PA1 the known apparatus can limitedly be applied as regards to the shape and character of the master cams that can be machined, e.g. the profiles marked with heavy lines in FIG. 2 can not be machined, where the different profile sections follow each other with a large variation .DELTA.R of the radius at a small central angle .DELTA..alpha. and/or the profile sections are repeated two or more times along the generatrix. PA1 The profile of the machine surface will be involute by coupling together the work piece and the main headstock in a spatial arrangement according to the known apparatus, but with a gear drive, with a Kinematic chain comprising elements transforming angular displacement into rectilinear displacement, e.g. gears and a gear rack. PA1 The problem arising from the large changes in radius related to small central angles and the problem of pitching arising from the twice or several times occurring repetitions of the active surfaces can be settled by connecting serially a unilateral claw clutch and a non-reversing free wheel and by arranging them in a Kinematic chain connecting together the work piece and the basic slide in such way, that the clutch and the free wheel operate in a reverse mode. Accordingly, the clutch opens in a direction of rotation in which the free wheel locks, and reversely. PA1 it facilitates the highly productive and accurate machining of steer cams, mainly of the active surfaces of brake spanners, PA1 it facilitates the inexpensive, accurate and highly productive machining of active surface which otherwise can be produced only by forging or by copying shank cutting of low level (e.g. the machining of the brake spanner according to the referenred Hungarian Patent Specification), PA1 it facilitates a cutting of active surfaces of e.g. brake spanners which is competitive with finish forging.
Brake spanners of arbitrary profile, e.g. the active surface of the brake spanner according to the Hungarian Patent Specification 175.132 cannot be machined by the known apparatus. The profile of the active surface of said brake spanner, as an involute, is not derived from the base circle the center of which coincides with the axis of rotation of the brake spanner, but from a base circle the center of which is displaced from it in a given direction. This embodiment counterbalances the decrease in the braking force during operation. In FIG. 3 the involute profile of a concentric and an eccentric base circle are marked with heavy and dash lines, respectively, where the brake spanners operate identically and are mounted in the same position.
If the involute derived from the base circle of displaced center, marked with a dash line could be restored when the active surface of the outworn brake spanner is renewed then during the improvement an active surface of higher value of utility could be produced than that of the brake spanner when it was originally manufactured. The known apparatus does not afford such facility in which its main disadvantage lies. A further disadvantage is that only a single work piece can simultaneously be machined on the known apparatus which is disadvantageous in respect of productivity.